A mobile communications system refers generally to any telecommunications system which enables wireless communication when users are moving within the service area of the system. A typical mobile communications system is a Public Land Mobile Network (PLMN). Often the mobile communications network is an access network providing a user with wireless access to external networks, hosts, or services offered by specific service providers.
One special feature offered in mobile communications systems by PMR systems is group communication. Conventionally group communication has been available only in trunked communications systems, such as Professional Radio or Private Mobile Radio (PMR) systems which are special radio systems primarily intended for professional and governmental users, such as the police, military forces, oil plants. However, the group communication is now becoming available also in public mobile communications systems. The term “group”, as used herein, refers to any logical group of three or more users for participating in the same group communication, e.g. a speech call. The same user may be a member of more than one group communication group. Typically, the members of the group communication group belong to the same organization, such as the police, the fire brigade, a private company, etc. Also, typically, the same organization has several separate groups, i.e. a set of groups.
Group communication has conventionally been based on predefined, rather static groups. In the first generation of conventional PMR systems, communication was based on access to physical radio channels. Radio channels were physically installed into mobile radio stations, the use of each radio channel was based on an agreement between the users. As it was not possible to dynamically introduce new radio channels into mobile radio stations, it was in principle possible to dynamically modify the agreement concerning which kind of traffic to be conducted on which radio channel. In practice this was difficult to achieve, and furthermore it was not possible to exclude users from having access to channels if they once were installed. To allow differentiation between groups on same physical channels, various selective call methods were introduced. Even these were based on the selective call information that was physically programmed into a mobile station. More dynamic approaches did not appear until more sophisticated signaling methods, which made it possible to exchange information in both directions between mobiles and a system control unit, were adopted. Special signaling messages could then be adopted to carry the regrouping information. Problems with reliability were common, because the loss of a message resulted in incomplete group membership.
In older and simpler systems the group membership was known only to the mobile stations. In more recent systems the group membership is known to both the system and the mobiles. The latter case provides many functional advantages (such as allowing the system to allocate resources only when being actually required by a group member) but makes the dynamic re-grouping technically more demanding and complex. The information in the system and the mobiles must match with one another. This is usually achieved by arranging the radio system to hold a master copy of the group information. This works well as far as it is enough to perform group management from a fixed terminal connected to the network infrastructure.
Various ways have existed for a long time to create new talk groups and/or modify group membership more dynamically. A need has particularly arisen to create and manage groups from mobiles. This has led to a two-phase solution: the user from the mobile has to communicate, using specialized signaling, with the system control unit, which stores the information, creates the group and distributes the information to other mobiles, again using specialized signaling. The problem with such a dynamic talk group functionality has been that the technical implementation thereof has been complex, and that said functionality has been difficult and unreliable from the user's point of view.
In addition to conventional PMR users, group communication is also attractive to users of modern public cellular networks. For example, private persons might want to participate in talk groups, such as hobby groups, sport groups, etc. Small business users may also use the group communication feature for a more frequent job-related communication during a working day within the same work group, either within the company or a business community.